//:protobuf_java_util: discontinue JDK 7 compatibility support.

6bbd56dfd9 removed the -source 7 -target
7 javacopts from the //java:{core,lite} targets in order to improve
compatibility with more recent versions of javac. (see that commit for
more discussion of motivation.) but the commit left -source 7 -target
7 javacopts on the related //java/util target. i've confirmed with the
commit author that this was just an oversight, so remove these flags
as well.

java/util/BUILD

@@ -5,10 +5,6 @@ java_library(
     srcs = glob([
         "src/main/java/com/google/protobuf/util/*.java",
     ]),
-    javacopts = [
-        "-source 7",
-        "-target 7",
-    ],
     visibility = ["//visibility:public"],
     deps = [
         "//external:error_prone_annotations",